Fix generated declaration types

[itosa-kazu]

Summary

• point the top-level package types field at the generated declaration file
• add missing markdown-it and Token typedefs used by the generated Renderer type
• avoid emitting markdown-it .default references that fail for consumers using @types/markdown-it

Verification

• npm run check
• npm test
• npx eslint src/options.js src/plugin.js src/renderers.js
• npm pack, install the local tarball into a clean TypeScript project, and run npx tsc --noEmit

The current published package fails the same consumer TypeScript test with missing Token/MarkdownIt names and the markdown-it .default type reference.

[runarberg]

Closing: I do not accept drive-by PRs from clankers hoping to catch a bounty.